The present invention generally relates to devices and methods for generating and delivering continuous positive airway pressure therapy to patients, such as infants. More particularly, the present invention relates to a variable flow, nasal continuous positive airway pressure device, system, and method with improved work of breathing characteristics.
Continuous positive airway pressure (CPAP) therapy has been employed for many years to treat patients experiencing respiratory difficulties and/or insufficiencies. More recently, CPAP therapy has been advanced as being useful in assisting patients with under-developed lungs (in particular, infants and especially premature infants or neonates), by preventing lung collapse during exhalation and assisting lung expansion during inhalation.
In general terms, CPAP therapy entails the continuous transmission of positive pressure into the lungs of a spontaneously breathing patient throughout the respiratory cycle. CPAP can be delivered to the patient using a variety of patient interface devices, for example an endotracheal tube. With infants, however, it is more desirable to employ a less invasive patient interface device, in particular one that interfaces directly or indirectly with the nasal airways via the patient's nares (e.g., mask or nasal prongs). Such systems are commonly referred to as nasal continuous positive airway pressure (nCPAP) systems.
In theory, the CPAP system should deliver a constant, stable pressure to the patient's airways. With conventional, ventilator-based CPAP devices, a relative constant and continuous flow of gas (e.g., air, O2, etc.) is delivered into the patient's airways, with this airflow creating a pressure within the patient's lungs via a restriction placed on outflow from the patient. Unfortunately, this continuous flow can have an adverse effect on the patient's respiratory synchrony. More particularly, the patient is required to exhale against the incoming gas, thus increasing the patient's work of breathing. Control valves can be employed to better accommodate inspiratory and expiratory stages of a patient's breathing (e.g., controlling gas flow into the system and/or altering an extent of restriction to outflow from the system). However, for many patients, especially infants, the ventilator approach is less than satisfactory as the patient's required work of breathing remains quite high. That is to say, it is essentially impossible for a control valve system to accurately replicate the actual respiratory cycles experienced by the patient, such that the patient will consistently be required to exhale against the high momentum, incoming gas, as well as against the resistance of the control valve(s). For an infant with under developed lungs, even a slight increase in the required work of breathing may render the CPAP system in question impractical.
More recently, nCPAP systems have been developed that incorporate a variable flow concept in combination with separate channels for inspiratory and expiratory gas to and from the patient. When the patient inhales, the incoming gas takes the path of least resistance and is directed to the patient's airways. Upon expiration, the gas again takes the path of least resistance and goes out an exhalation or exhaust tube, thus reducing resistance during the expiratory phase. For example, the Infant Flow™ system, available from Viasys Healthcare, Inc., of Conshohocken, Pa., includes a variable flow CPAP generating device (or “CPAP generator”) that purportedly causes the direction of the supplied gas to change with the infant's breathing patterns while maintaining a constant pressure throughout the respiratory cycle. The Infant Flow CPAP generator forms two conduits (one for each of the patient's nares), and an exhaust tube. Gas is directed into each respective conduit via an injector nozzle. The momentum of the gas jet acting over the area of the conduit creates a positive pressure inside the patient's lungs, in accordance with known jet pump principles. To accommodate expiratory flow from the patient, the generator relies upon what the manufacturer's literature characterizes as a “fluidic flip” effect. More particularly, the expiratory airflow from the patient applies a pressure onto the incoming flow (within the conduit) from the injector nozzle. It has been theorized that due to the coanda effect, the expiratory airflow causes the nozzle flow to deflect, thus triggering a fluidic flip of the airflow from the nozzle. As a result, fluid flow from the nozzle, as well as the expiratory airflow, readily proceed to the exhaust tube, thus reducing the patient's required work of breathing. While highly promising, current nCPAP products incorporating the “fluidic flip” approach may be less than optimal. For example, the injector nozzle airstream has a relatively high momentum that may not be easily overcome by the patient's expiratory breathing, especially with infants.
In light of the above, a need exists for an improved nCPAP device, system, and method.